Heretofore, in many cases there have been used zircon lead titanate (PZT) ceramics as materials for a piezoelectric vibrator of ultrasonic transducers. Those piezoelectric ceramics are, however, disadvantageous in: (i) an acoustic matching layer requires an ingenious design when used for diagnostic purposes, because acoustic impedance is significantly larger than that of the human body, (ii) a dielectric constant is significantly large and hence a piezoelectric voltage constant g is so small that high voltage can not be produced upon receiving ultrasonic waves, and (iii) it is difficult for those ceramics to have a curvature fit for the shape of the human body. To solve such problems, there have been proposed so-called piezoelectric composites in which polymers are compounded with piezoelectric substances. As one example, Newnham, et. al. in the United States reported that such a composite structure is effective where a number of PZT poles 12 are buried in a polymer 11 as shown in FIG. 1 (see "Material Research Briden", Vol. 13, pp. 525-536 (1978)). In fact, the composite structure of PZT and polymers, such as silicon rubber or epoxy, results in a material having low acoustic impedance and a large piezoelectric voltage constant g.
In those piezoelectric composites, their piezoelectric characteristics are greatly varied depending on the volume ratio of the piezoelectric substance to the polymer. This is described in detail in the above reference. But it is expected that the piezoelectric characteristics also varied depending on the size and arrangement of piezoelectric poles even with the same volume ratio of the piezoelectric substance.